Laser designator and pointing systems are widely used systems which use the spatial optical detection of a reflected laser beam in the imaged field of view in order to pin-point the target to be designated. In low illumination situations, this task is comparatively simple since the laser illuminated target stands out well from the background. However, the main challenge of laser pointing and designator systems is to be able to recognize the laser signal reflected from the field of view being surveilled, while the field of view is bathed in daylight background illumination. The intensity of the sun's illumination may be orders of magnitude larger than that of a low average power laser beam directed into the field from a distance, making detection of the laser beam reflection difficult.
Current laser pointing and designator systems generally use a pulsed solid state laser, often a Nd:YAG laser, for illuminating the target. The pulse width is typically on the order of 20 to 50 ns, and for a laser having an average power of a few watts, this means that the peak power of the pulses is of the order of many tens of megawatts. The solar radiation deposits approximately 1 kW per square meter. The use of a wavelength discriminating filter in order to limit the detected background to the wavelength range of the laser pulses, may reduce this flux by a factor of some tens or even one hundred. Consequently the peak power of the laser beam may be a factor of tens of thousands larger than the solar radiation detected in the wavelength window admitted by the system filter. Consequently, it is a comparatively simple matter to detect the flash of a laser pulse from the illuminated background of the field of view. Furthermore, a gated detection system synchronized to the short laser pulses, may be used, such that only the reflected light of the pulses is detected, thereby rejecting the majority of the background shot-noise.
The spatial position of the reflected laser pulse is typically determined by use of a quadrant photodiode, as shown in FIG. 1. The received light is imaged onto the quadrant detector thus providing directional information as to where the spot is in relation to the center of the imaged field of view. In more advanced versions of quadrant detectors, a comparatively large spot can be used, and an algorithm comparing the output of the various quadrants is used to approximately determine the location of the reflected laser beam in the actual field of view. The lack of more detailed information about the position of the laser spot in the field of view has led to guidance systems for munitions homing onto the laser designator, which cause the projectile to perform a number of decreasing oscillations during its path towards the target. One such system is described in U.S. Pat. No. 6,097,481 assigned to the Northrop Grumman Corporation, and known as the “Bang Bang Laser Spot Tracker”.
Various schemes of temporal coding such as changing the spaces between the pulses, can be used, both in order to discriminate between different designators, and in order to avoid simple countermeasures from interfering with the system.
This technology requires a high peak power pulsed laser, in order to enable a sufficient level of energy for detection above the background level. Such lasers generally have a number of disadvantages, in that they have complex, fragile structures, comparatively large dimensions, limited reliability and lifetime, and are a high cost item. As a result, laser designators and pointers are not widely used, and in those cases where use is made of them it is generally by dedicated personnel and units
There therefore exists a need for a laser designation and pointing system, which overcomes at least some of the disadvantages of prior art systems and methods, and in particular, capable of operating in full sunlight, yet without the need to use a high peak power, pulsed laser source. In addition there exists a need for a substantially lower cost laser designation and pointing system, such that wider use can be made of such systems.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.